1. Field of the Invention
The present invention relates to the prevention of scale formation and enhancement of oil recovery in carbonate oil reservoirs, and particularly to a chelating fluid formed from a polyamino carboxylic acid chelating agent in brine and the usage thereof in water injection systems of carbonate oil reservoirs.
2. Description of the Related Art
Scale deposition is one of the most serious concerns in oil reservoirs, particularly in water injection systems therefor. Scale problems are particularly prevalent in systems which combine two incompatible types of water, such as sea water and formation brines. Two types of water are considered to be incompatible if they interact chemically and precipitate minerals when mixed. Typical examples include sea water with high concentrations of sulfate ions (at least 4,000 ppm) and formation waters, with high concentrations of calcium, barium and strontium ions (often more than 30,000 ppm). Mixing of these waters may cause precipitation of calcium sulfate, barium sulfate and/or strontium sulfate.
Scale formation in surface and subsurface oil and gas production equipment is not only a major operational problem, but also a major cause of formation damage in both injection and production wells. Scale formation may cause equipment wear and corrosion, along with flow restriction which results in a decrease in oil and gas production due to the excessive pressure drop. Scale deposition further restricts the oil and gas flow by decreasing the area available for flow (by a decrease of the flow pipe's diameter) which, in turn, causes an increase in the friction pressure losses. The latter consideration affects the flowing bottom hole pressure and, consequently, the outflow performance of the well. The lowered outflow performance lowers the well's draw down and decreases the overall deliverability of the well. Scale deposits often form at the tops of wells, requiring removal of the associated pipes and tubing, which generates high operational costs and temporary work stoppages.
Scale control is typically performed as a two-step treatment involving first removal of the precipitated scale, and then prevention of its reformation by chemical inhibitors. At present, there is no effective single stage treatment that will both remove and inhibit scale precipitation in oil and gas reservoirs during the process of water injection in enhanced oil recovery or pressure maintenance processes.
Typical scales formed in such environments often include calcium sulfate. These deposits can typically be removed chemically. However, scale composition frequently changes during the production history of the well, causing many scales that are initially subject to chemical removal treatments to become very difficult to be removed by subsequent chemical treatment. There are many chemicals available that will prevent scale deposition, however most will not remain in the formation long enough to make them economically feasible as inhibitors.
Thus, a chelating fluid for enhanced oil recovery in carbonate reservoirs and a method of using the same addressing the aforementioned problems is desired.